The disclosed systems generally relate to the application of electrical motors/generators and more particularly to homopolar machines including superconducting windings.
Historically, most superconducting electric machines have included a superconducting field coil installed on the rotor. The superconducting coil is maintained at a temperature approaching zero Kelvin using a continuous supply of cryogenic fluid such as, but not limited to, helium (He). If a high temperature superconductor (HTS) is used in fabricating the field coil, a cryogenic fluid such as nitrogen (N2) may be used to achieve superconducting temperatures. The cryogenic fluid is typically supplied to the superconducting field coil from a stationary cryocooler through a transfer coupling that is coupled to one end of the rotor. The transfer coupling channels the cryogenic fluid from a stationary portion to a rotating portion on the rotor. The cryogenic fluid is then routed through a cooling loop thermally coupled to the superconducting field coil and then back to the transfer coupling for return to the stationary cryocooler.
The superconducting field coil is subjected to thermal stresses, centrifugal stresses, and is provided with an electrical connection through the rotor to power the superconducting field coil. Accordingly, designing, fabricating and operating such a rotor may be difficult. For example, the superconducting coils, especially HTS coils, may be sensitive to mechanical strain. Specifically, because the coils are coupled to the rotor, the coils may be subjected to centrifugal forces that may cause strains and degrade the performance of the superconductor. In addition, because the coil is maintained at a cryogenic temperature, an elaborate support system may be needed to maintain the coil in position against the centrifugal forces while preserving the integrity of the thermal insulation between the coil and the parts of the rotor at ambient temperature. Further, the performance of these machines limits the application of the same.
Problematic areas where an electromechanical applications of energy conversion are required are portable military auxiliary power for land, air or sea based platforms, where a portable turbine drives the generator. The high power density sought requires a high speed machine, with a rugged rotor. An electric ship drive motor and generator provide power for propulsion, which has high power density, high efficiency and can be driven directly or through a gearbox. Other applications include: Ship auxiliary power (High power density, high efficiency requirement); Hydrogenerator (High efficiency required and usually low speed); Wind generator (high power density required with very low speed); Frequency shifter (generator with controlled low frequency ac in field winding to generate constant frequency power for varying turbine speed).